This invention relates to an automatic derailleur shifter system for a vehicle which is a bicycle which includes a pedal powered chain wheel which drives through a chain one of a set of driven sprockets attached to a drive wheel of the vehicle.
Most bicycle gear systems now use a derailleur system which includes an idler gear mounted upon a spring system which tensions the chain and acts to move the chain to a selected one of a set of driven sprockets. The derailleur system has become over the years very finely tuned and now generally includes also a locator arrangement which ensures that the changing mechanism is properly positioned at one of the driven sprockets of the set in preference to intermediate positions. In addition the gear chain system often includes two or even three chain wheels at the drive end which are attached to the pedal arrangement together with a changer system which moves the chain across between the drive sprockets. Some systems have now reached the complexity of having twenty one possible change combinations provided by three drive sprockets at the forward end and seven driven sprockts at the rear hub. The systems are changed by a simple lever arrangement which pulls upon a cable to move the front chain system or the derailleur system at the option of the rider. The rider is therefore faced with a wide variety of possibilities and only the most experienced and enthusiastic of riders can control the gear system sufficiently to maintain the pedal pressure or torque within the relatively narrow range which is preferred.
Attempts have been made over many years to provide an automatic change system which detects the amount of torque or pressure applied to the pedals and then changes the gear ratio automatically to attempt to maintain the torque within relatively narrow band.
Various arrangements are shown in U.S. Pat. Nos. 3,929,025 (Perry); 4,599,079 (Chappell); 3,831,978 (Dunder); 4,781,663 (Reswick); 4,061,046 (Lang); 4,343,613 (Leiter) and 3,769,848 (McGuire).
Most of these arrangements provide highly complex detection systems including additional elements mounted on the bicycle for detecting the tension in the chain or additional elements for detecting the pressure at the pedals.
The McGuire patent provides a relatively simple device in which the sprocket set is caused to move axially of the rear axle by a screw thread coupling on an inner surface of the sprocket set and outer surface of the axle. On the face of it this arrangement is relatively simple but it requires a significant modification of the structure of the bicycle so that the sprocket set is arranged on the outside of the forks rather than on the inside as is the conventional arrangement. The device cannot therefore be simply attached to an existing bicycle and major modifications to the bicycle or a special bicycle are necessary.
In any event none of the devices proposed has had any significant impact on the marketplace and basically no automatic shifting arrangement is currently available.